ALSA: als4000 - Code clean up

- use specs-derived register name enums instead of open-coded numeric
values, for better understanding of things
- fix naming confusion ("gcr" was _NOT_ the GCR register stuff, but
  instead the io _base_ which has multiplexed _access_ to GCR config
  space, at _sub_ registers 0x08 and 0x0c)
- add FIXME comments about a race condition and MPU401 features

Signed-off-by: Andreas Mohr <andi@lisas.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>